1. Field of the Invention
This invention relates to precision voltage reference sources, and more particularly to variable precision voltage reference sources.
2. Background Information
A power supply for a computer requires a precision reference voltage for the power component of the power supply. The reference voltage is often referred to as a xe2x80x9cset pointxe2x80x9d voltage. The set point voltage has a number of requirements. For example, the computer requires accuracy of the set point voltage of better than xc2x10.5%. Secondly, modem computers require smaller voltages, some set point requirements being less than one volt. At low voltage outputs, accuracy of the set point is dominated by both the use of resistors, and by op-amp errors such as individual device dependent offset voltages. Further, some computers now require over 30 different voltage set points, and each has an accuracy requirement. The computer requests voltages by use of Voltage Identification Codes (VID codes), and may output a different VID code for each voltage level which it requests from the power supply.
Even further, precision voltage settings for a reference voltage for a DC/DC converter need to be adjusted to compensate for the temperature dependence of the reference voltage, resistor values, and offset errors of operational amplifiers, in order to maintain a constant voltage output to the computer from the DC/DC converter as the temperature of the unit varies.
Besides computers, an increasing number of electronic devices require power sources at different voltages. Particularly, power is required at voltage levels which are selected by a processor generating VID code commands. Further, it is important that power be delivered at the voltage specified by the VID code.
The traditional solutions for achieving the above design requirements of supplying power at a voltage specified by a VID code are to use expensive precision resistors, that is resistors of at least 0.1% tolerance, or better, and to have manual adjustment facility within the precision reference power supply. Also, expensive low offset operational amplifiers need to be used, and these may require manual unit to unit trimming in order to obtain the desired output voltages. Further, the use of positive temperature coefficient components is sometimes necessary in order to compensate for the temperature dependence of the voltage reference, and other error sources.
A power supply having better means for supplying the multiple accurate set point voltage requirements of modem electronic design is desirable.
A power supply has a digital input to receive Voltage Identification codes (VID codes). A microprocessor within the power supply reads the VID codes and outputs, in response to the VID codes, a digital to analog converter code (DAC code). A digital to analog converter receives the DAC codes, and outputs a reference voltage. A DC to DC unit receives the reference voltage and also receives power from a standard power supply, and the DC to DC unit generates output power at a selected output voltage in response to the reference voltage. A test stand measures the output voltage, and the test stand adjusts the DAC code generated by the microprocessor in response to the VID code in order to supply power at the desired output voltage by the power supply, in response to a desired input VID code. The test stand adjusts the DAC code generated by the microprocessor in response to the VID code so that a particular VID code causes the DC to DC unit to generate output power at the voltage specified by the VID code.
A precision reference voltage is generated by the digital to analog converter in response to the DAC code, and the precision reference voltage is applied to a DC to DC converter which receives power from a standard power supply.
In an exemplary embodiment of the invention, the computer generating the VID codes is the load receiving power at the voltages specified by the VID codes.
In a further exemplary embodiment of the invention, a pulse width modulation unit is used for the digital to analog converter to generate the precision reference voltage.